Integrated circuits generally require a temperature-sensing circuit element to sense a local temperature so that protection for the circuit can be provided against an overtemperature condition, especially in automotive power applications where high temperatures under the hood of a car can be experienced. A thermal shutdown circuit prevents the integrated circuit from destruction under a fault condition, for example, when the output of the integrated circuit is continuously short-circuited. The temperature-sensing device is generally specified to be quite precise, since the difference between an acceptable operating temperature, for example, a device temperature of 150° C., and a maximum allowed silicon temperature, for example, 200° C., is often not that large in view of the difficulty of accurately reproducing temperature-sensing devices in silicon and other semiconductor materials.
A conventional way to sense temperature with a silicon device is to sense a bipolar transistor base-emitter junction voltage when the transistor is supplied with an accurately controlled collector current. An accurately controlled current can be produced by a current mirror using techniques well known in the art. The controlled current flows through the base-emitter junction that is sensed. The base-emitter junction voltage can be detected by a comparator with an input terminal coupled to another reference voltage. Sensing the junction voltage when it carries a known current provides the underlying mechanism to implement thermal protection. The base-emitter junction voltage with changing temperature has a slope that is approximately −2 mV/K, with excellent linearity for a fixed collector current over the whole range of temperatures experienced in an automotive environment.
However, to provide a sufficiently accurate indication of temperature for automotive and other temperature-sensitive applications that is independent of integrated circuit manufacturing process variations requires a substantial collector current in the temperature-sensing bipolar transistor. The high collector current increases the total current consumption of the integrated circuit to be protected. To diminish the impact of the large collector current on the integrated circuit quiescent current, the biasing current for the temperature-sensing transistor collector has been decreased in recent device designs. The result of the reduced collector current is imprecision of the sensed temperature, or the need for more controls and associated costs for a manufacturing process for the device. The quiescent current of an integrated circuit is generally a device-specified parameter that is generally required to be a low value, particularly in automotive and other battery-powered applications.
Other examples are encountered in which there is a need to accurately sense a varying level of a physical parameter, such as a strain, a pressure, or an electrical flux, with an electronic device. Particularly in those instances where the sensing instrument is portable or remotely powered, the need to accurately signal a high or low level of the physical parameter with minimal electrical drain is an important design consideration.
Thus, there is a design trade-off between the accuracy of sensing a physical parameter such as temperature and device dissipation in conventional semiconductor parameter-sensing arrangements. There is a need for a process and related method to provide an accurate indication of a physical parameter such as a local temperature for device protection without incurring a substantial power dissipation penalty that avoids the disadvantages of conventional approaches.